1. Field of the Invention
The present invention relates to a technology for controlling pressure application force of a pressure-applying device for use in an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses that support color image output, such as color copiers and color printers, are widely used. Such color image forming apparatuses can be single drum type or tandem type. The single drum type image forming apparatus has one photosensitive member functioning as an image-carrying member, and a plurality of developing devices of different toner colors disposed around the photosensitive member. The toner of each color from each of the developing devices is made to adhere to the photosensitive member to form a composite toner image. The composite toner image is transferred from the photosensitive member to a transfer sheet functioning as a recording medium. On the other hand, the tandem-type image forming apparatus has a plurality of photosensitive members arranged in a row in a conveyance direction of a transfer sheet, with a separate developing device for each toner color disposed in the vicinity of each photosensitive member. The individual toner image of each color is formed on the respective photosensitive member, and each toner image is sequentially superimposed onto an intermediate transfer unit by primary transfer, so that a composite image is formed. The composite image is eventually transferred to the transfer sheet by secondary transfer, so that a full color image is obtained.
The single drum type image forming apparatus is compact and less expensive compared to the tandem-type image forming apparatus. However, it is difficult to increase the speed of image formation in the single drum type image forming apparatus because image formation has to be repeated several times to obtain a full color image. On the other hand, the tandem-type image forming apparatus is less compact and more expensive but has the advantage of fast operation speed of image formation.
Generally, color image forming apparatuses use toners of four different colors and therefore image formation has to be repeated as much as four times in the single drum type image forming apparatus to obtain a full color image.
In the tandem-type image forming apparatus, the transfer of the toner images formed on the photosensitive members can be by a direct transfer method or an intermediate transfer method. Explanation about the direct transfer method is given with reference to FIG. 11. Four transfer units 91 are provided for each of four photosensitive members 90 corresponding to each toner color. Each of the transfer units 91 sequentially transfers the image formed on each of the photosensitive members 90 to a transfer sheet S conveyed by an endless transfer sheet conveying belt 92 that is driven to move in a predetermined direction at a predetermined speed by a belt driving device or a transfer sheet driving system. Explanation about the intermediate transfer method is given with reference to FIG. 12. Each of the transfer units 91 sequentially transfers by primary transfer an image formed on each of the photosensitive members 90 to an endless intermediate transfer belt 93, so that a composite color image is obtained. The intermediate transfer belt 93 functions as the intermediate transfer unit and is driven to move in a predetermined direction at a predetermined speed by the belt driving device or the transfer sheet driving system. A secondary transfer unit 94 transfers by secondary transfer the composite image on the intermediate transfer belt 93 at once to the transfer sheet S. The intermediate transfer unit can be in the form of a roller instead of a belt.
When a roller is employed as the transfer units 91 or the secondary transfer unit 94 in a single drum type image forming apparatus, to prevent density unevenness during image transfer, pressure application force to the transfer unit along its axial direction should be uniform. A spring can be provided as a pressure-applying member at either end of the shaft of the transfer unit to apply pressure to the two ends of the transfer unit. However, pressure application force is likely to be unstable by this method.
Japanese Patent Application Laid-open No. 2000-122445 discloses a technology for controlling pressure application of a pressure-applying member provided on a drive-force receiving side of a transfer unit. Specifically, the pressure application force is set higher by a force component of driving force that acts in a normal direction to the teeth surface of drive-force transmitting gears, in an expansion-contraction direction of a spring. Therefore, it is possible to compensate for decrease of the pressure application force on the drive-force receiving side caused by the component force in an expansion-contraction direction of the spring. Thus, a uniform pressure can be applied to the transfer unit along its axial direction.
However, some of the disadvantages of the conventional technology are increased number of components and difficulty in determining an optimum load value for the spring. It is difficult to determine an optimum load value for the spring due to the following reason. The driving torque is constant when there is no transfer sheet S between the roller-type transfer unit (hereinafter, “transfer roller”) and an opposing roller in pressure contact with the transfer roller and when there are no variations in the component (such as roller diameter and installation position). However, the driving torque is not constant if a thick paper is used as a transfer sheet or if there are variations in the component precision. In other words, during operation, the driving force acting in the normal direction of the teeth surface of the drive-force transmitting gear varies. Therefore, stable pressure to the transfer unit along the axial direction cannot be achieved by merely changing the load value of the spring. The inability to maintain constant pressure application force along the axial direction between the rollers results in image density unevenness due to faulty transfer.
The mechanism described above is explained with reference to a pressure-applying device employing a conventional drive-force transmission method. FIG. 13 is a schematic diagram of the pressure-applying device employing the conventional drive-force transmission method. A reference numeral 95 in FIG. 13 denotes a secondary transfer roller and A reference number 96 denotes an opposing roller. The secondary transfer roller 95 is rotatably supported by a transfer unit (not shown). The transfer unit is rotatably supported at a rotation center AO in the main body of the image forming apparatus, is in pressure contact with the opposing roller 96, and is biased upwards by a pressure-applying spring (not shown) disposed at a pressure application point A1. A gear 97 is provided coaxially with the secondary transfer roller 95. A first idle gear 98A is engaged with the gear 97, a second idle gear 98B is in turn engaged with the first idle gear 98A, and a driving gear 99 is engaged with the second idle gear 98B. The driving force from a driving motor (not shown) is conveyed to the secondary transfer roller 95 via the gear 97, the first idle gear 98A, the second idle gear 98B, and the driving gear 99, causing the secondary transfer roller 95 to rotate.
If F1 is reactive force at the driving-gear end, F2 is reactive force at the non-driving-gear end, T is driving force, W is self-weight of the transfer unit, and P is pressure application force of the pressure-applying spring, the pressure-applying mechanism model can be given by the following expression based on the principle of moment equilibrium.
At the driving-gear end,F1×cos θ1×L1=P×sin θ2−W×L3−T×L4Therefore,F1=(P×sin θ2×L2−W×L3−T×L4)/(L1×cos θ1)  (1)
At the non-driving-gear end,F2×cos θ1×L1=P×sin θ2×L2−W×L3Therefore, F2=(P×sin θ2×L2−W×L3)/(L1×cos θ1)  (2)
Thus, it can be surmised from expression (1) that the driving force has an effect of weakening the reactive force F1. The variation in the reactive force F1 can be reduced by increasing the load of the pressure-applying spring to the extent to which the reactive force F1 is weakened by the driving force T. However, it is difficult to maintain the reactive force F1 constant during operation because of the variation of the driving force T due to variation in the component specification or the presence of the transfer sheet S between the rollers.